1. Field of the Invention
This invention relates generally to the field of electronic device user interfaces and authorization techniques, and more specifically to the field of fingerprint imaging sensors and touch screen display apparatuses.
2. Necessity of the Invention
Modern electronic devices have developed a myriad of functionalities and associated user interfaces. Many electronic devices use a display screen, such as a monitor or display apparatus, to provide feedback to the user. Handheld devices, such as the personal digital assistant and the cell phone, have an important user interface constraint—form factor. In both devices, manufacturers desire to minimize the size and weight of the device; as one means to accomplish this, the display is small and buttons are placed close together.
In recent years, manufacturers of many electronic devices have substituted touch screen technology for the traditional display. Touch screens have the same appearance and style of a traditional screen, but have the added ability to determine the location of applied pressure. This allows individuals to use a stylus in a similar manner as a person uses a mouse to point to icons on a monitor—the individual may touch the screen at the location of a particular icon. Software running on the device determines the location of the touch and determines the associated software function, such as opening an address book. Because the additional button control interface can be eliminated, manufacturers can make the display larger and simpler to use.
As the functionality of electronic devices expands, individuals may wish to protect certain data stored within the device. For example, the owner of a personal digital assistant may choose to use his PDA to send and receive private e-mail. If the data is particularly sensitive, a simple password or PIN combination may not be considered adequate security and the individual may desire to use biometric authentication on the device. The most common form of biometric authentication, fingerprint scanning, requires a hardware module that is typically the size of a postage stamp. On a device where size and weight are limited, the addition of this module can be costly.
Digital Fingerprint Capture Technologies
There are three common types of fingerprint capture technologies: optical, capacitive, and ultrasonic. Each of the three technologies combines its associated hardware capture mechanism, which varies from type to type, and typically a software or firmware controller. This controller is often responsible for analyzing the captured image, extracting minutia points, and creating a final template. Minutiae are points that represent all of the unique characteristics of a fingerprint—one example is the location of an intersection of ridges or valleys in the print. A template is typically composed of thirty minutiae and can be used to uniquely identify a fingerprint. This allows the scanner or other storage device to store only the requisite data points without storing the entire image.
Of the three types of fingerprint capture technologies, optical scanners are the oldest and most common, and they are composed of a glass or plastic plate with a light source and a charge coupled device (CCD) beneath. The light source is typically an array of light emitting diodes (LEDs), and the CCD is an array of light-sensitive diodes. When the finger is placed on top of the plate, the LEDs illuminate the finger and each diode of the CCD records the light that touched it, creating an image in which the ridges are dark and the valleys are light. Optical scanners are fairly resistant to temperature fluctuations, and can provide an image quality of approximately 500 dots per inch (dpi). One major concern of this technology is that latent prints—“left over” fingerprints on the plate—can cause a superpositioning effect and create error. Additionally, these types of scanners are susceptible to “gummi bear attacks”, in which a fingerprint is lifted from a glass or other object, placed on a pliable and sticky material, such as a gummi bear, and can provide a false acceptance. One other point of note is that the plate must be quite large; this creates ease of use but may take unavailable real estate on a board.
Capacitive sensors are much newer than optical scanners, and are composed of an array of cells; each cell has two adjacent conductor plates, which are embedded within an insulating layer. The insulating layer is typically a glass plate. When the finger is placed on top of the insulating layer, it creates a subsequent electric field between the finger and the conductor plates, creating capacitance. Because the surface of a finger is a succession of ridges and valleys, the electric field varies over the face of the finger as the distance from the plate to the finger varies. The capacitance or voltage may be determined from the electric field, and is commonly translated into an 8-bit grayscale image with approximately 200 to 300 grid points in both the x- and y-plane. This creates more detailed data than the optical sensor. Capacitive scanners are typically smaller than optical sensors because the cells are composed of semiconductor devices, rather than a CCD unit.
While capacitive scanners are cheaper and smaller than optical sensors, their durability is unknown due to their short time in use, and the small size can make it more difficult for an individual to enroll and authenticate properly. Most fingerprint sensors use direct current (DC) coupling, although a few companies are beginning to use alternating current (AC) coupling to penetrate to the live layer of the skin. Because the capacitive scanner is dependent on the electric field and capacitance between a finger and the glass plate, the scanner cannot be fooled by the “gummi bear attack” as described above; the dielectric constant for the finger is much different from a gummi bear, and so the capacitance will vary significantly.
The most accurate but least common finger-scanning technology is ultrasound imaging. In this type of sensor, two transducers are placed on the x- and y-axes of a plate of glass—one each for receiving and transmitting—for propagating ultrasound waves through a glass plate; when the finger is placed on top of the glass, the finger impedes the waves and the receiving transducer can measure the alteration in wave patterns. This type of scanner is very new and largely untested in a variety of conditions, but initial results show promise for the technology. It combines the large plate size and ease of use of the optical scanners with the ability to pervade dirt and residue on the scanner, an advantage of capacitive scanners.
Touch Screen Technologies
Touch screens are quite similar to the fingerprint scanners described above. They recognize a finger pressure on the screen and typically calculate the center or peak point of the pressure. Current touch screen technologies fall under five different types of technology: analog resistive, capacitive, infrared, acoustic wave, and near field imaging. The analog resistive, capacitive and acoustic wave technologies are the most commonplace due to their clarity and endurance under a variety of conditions. Infrared is very sensitive to a light touch and may be impractical, while near field imaging is very new, suitable for very harsh conditions, and frequently cost-prohibitive. For these reasons only the first three technologies are examined in much detail. Similarly to the fingerprint scanning technology there is typically an associated software or firmware controller to perform requisite data analysis.
The analog resistive technology is composed of a glass plate and a plastic plate stacked over a flat-panel screen or display. Both the glass and plastic plates are coated with a transparent conductive material, such that the conductive material is sandwiched between the two plates. Tiny separator dots keep the two plates from touching under normal conditions, but when pressure is applied to the plastic plate, the dots move and the two surfaces come together to conduct electricity. An electronic controller instantly calculates the x- and y-coordinates, allowing resistive touch screen technologies to have very high precision and resolution. This also allows an individual to have relative freedom when selecting an object as a stylus; the individual may use a pen, finger, or other convenient utility.
Capacitive coupled technologies require the use of a conductive stylus—this may be a finger, but not a gloved hand because the cloth will prevent the conduction of charge. Capacitive technologies use a flat-panel display with a single glass plate resting on top. The glass plate is covered in a transparent metal oxide on the exterior surface; when the finger or alternate stylus comes into contact with the conductive surface; capacitive coupling occurs at the point of contact and draws electrical current. The controller registers the change in current and the x- and y-coordinates can be determined. As mentioned above, because the technology requires use of a conductive stylus, non-conductive surfaces will prevent the change in electrical current and will not have any effect on the touch screen. Furthermore, the exposed glass surface in this technology makes it susceptible to scratches and can inhibit correct operation of the screen.
Acoustic wave touch screens are more complicated than the capacitive and resistive technologies. There are two types of acoustic wave technologies: guided acoustic wave (GAW) and surface acoustic wave (SAW). Both use a single plate of glass placed on top of a flat-panel display, with a similar transducer arrangement as described above for the ultrasound imaging. GAW screens transmit a wave through the glass panel (using the glass as a waveguide), while SAW screens transmit the wave on the surface of the glass; in both technologies, transducers detect a dampening of the wave that occurs when pressure is applied to the glass, which is translated into x- and y-coordinates. Similarly to the capacitive coupled screens, SAW screens have stylus limitations; the stylus must be soft and able to absorb energy in order to dampen the wave, and are generally only practical in instances where the stylus is a finger. These types of touch screens also have the glass surface limitation described above.